Many radio systems have been made with different communication protocols. In such circumstances, there is a radio system that varies the system function by varying the software program and reconfiguring the hardware to implement the radio system function with plural systems on the common platform, which is generally called the software defined radio system. Many efforts toward the software defined radio system have been introduced with various treatises and so forth. As an example, the collective treatise B, Vol. J84-B, No. 7, pp 1112-1119 (2001/7) from the Japanese Electronic Information Communications Association describes the status quo and future of the software defined radio system.
In view of the flexibility produced by varying the software program and reconfiguring the hardware, it is preferred to make up the software defined radio system so as to perform main processing with software, while reducing as many analog circuits as possible and using digital circuits or processors. However, the operation speed of digital circuits, the throughput of processors, and the conversion speed of AD converters and DA converters have their limits. And there are problems in the price and the power consumption. Therefore, it is not realistic to implement all the functions of the software defined radio system with the software processing using digital circuits and processors.
Accordingly, the allocation of analog circuits and digital circuits becomes extremely important in the software defined radio system. It is very important to allocate the sampling unit that administers the conversion of analog signals and digital signals to which part of the radio system. That is, to perform the sampling in the RF band (radio frequency band), or to perform it in the IF band (intermediate frequency band), or to perform it in the base band (BB) will give great differences in the circuit scale of analog circuits, the circuit scale of digital circuits, the sampling frequency, and the sampling accuracy and so forth. Here, the sampling unit represents an AD (Analog to Digital) converter that converts an analog signal into a digital signal, a DA (Digital to Analog) converter that converts a digital signal into an analog signal, and a part made up with an anti-aliasing filter which becomes necessary with the above converters.
In the case of performing the sampling in the RF band, the radio system is made up with full digital circuits, becomes easy to reconfigure the hardware, and becomes a realistic software defined radio system that can easily conform it to any communication protocols of the radio system. However, the sampling frequency becomes some ten GHz in this case, and the AD converter and DA converter having such a high frequency are not realistic in the price and power consumption. Not only for the sampling frequency, has it required about 20 bits as the quantization bit number.
In the case of performing the sampling in the IF band, the sampling frequency can be reduced to a lower frequency in comparison to the sampling in the RF band. However, in view of the price and power consumption, it is not yet realistic in the radio system such as a mobile phone and a wireless LAN available to general users. The high-speed data communication as seen in the wireless LAN and the like is expected widespread in the future. However, since the high-speed data communication requires a wide base band, it is impossible to lower the IF band.
Assuming the sampling in the RF band or the IF band, there is another method of taking on the undersampling that performs the sampling with a sampling frequency not higher than the Nyquist sampling frequency. In this case, the input band width of the AD converter and the frequency accuracy of the sampling are required in the same level as the sampling in the RF band and the IF band. The sampling in the base band is regarded as a general makeup, and many software defined radio systems called the multimode wireless device take on this configuration.
The software defined radio system is required to transmit/receive the signals having different bandwidth for different protocols. In the sampling in the base band, there appears a necessity of sampling with multiple different sampling frequencies. In that case, the cutoff frequencies of the anti-aliasing filters have to be varied in correspondence with the multiple different sampling frequencies.
The JPA No. 300017/1993 describes a traditional method of varying the sampling frequency and the bandwidth of the anti-aliasing filter, in correspondence with the frequencies of the analog input signals. And the JPA No. 198257/2003 describes a traditional method of fixing the sampling frequencies and the cutoff frequencies of the anti-aliasing filters, and performing the rate conversion to the data sampled by the fixed sampling frequencies by means of the digital signal processing.